THIS IS WHAT YOU NEED TO KNOW ABOUT HOW BEARINGS ARE MADE:

WHY THERE'S NO SUCH THING AS A 1/4" BALL:

It seems that very few people actually know how ball bearings are made. For those of you looking for something better, this is important. In the manufacturing of bearings, there are three basic machining operations. Inner race, outer race and the balls themselves. It is possible to manufacture balls with repeatable dimensional consistency, but it is not possible to do the same with the races. This is due to a number of factors, including variations in the machines themselves, but include others such as wear on the grinding wheels and even variations in temperature. As a result, bearing manufacturers don't really try to maintain the same dimensions from run to run, and instead rely on different sized balls to assemble the bearing to the required tolerances. Bigger gap between the races? No problem, just use a slightly larger 1/4"ball. Looking for a "tighter" fit? Use a larger ball. The variation in ball sizes often are separated by millionth's of an inch, well beyond the ability to measure by most of us. So what does this mean to you? Remember the following:

• Three different bearings of the same size can have three different sized balls.

• Balls are not interchangeable from bearing to bearing.

• Accidentally putting just one ball that is larger than the rest can spell disaster!

• It is extremely difficult to measure the differences with ordinary micrometers.

• You cannot simply buy "a ball", either steel or ceramic and expect it to be the right size.
  

So if you are in the market for assembled ceramics, it's all about ball quality and the "fit". To do it right takes a healthy assortment of ceramic balls, which adds up to a sizeable investment quickly. Everyone starts with a pre-made bearing, and the steel balls are simply swapped out with ceramics. Now there are an infinite number of possibilities of inner and outer race/ball combinations and each will require one particular ball size to meet the required tolerances. So having only three of any size category simply won't cut it. It takes more than that. Ceramic balls are expensive (good ones anyway), and this is an easy way to cut costs. So the next time you hear about a failed ceramic horror story, consider the above. Don't ask the bearing maker what type of balls they use, instead ask how many 1/4" balls they have on hand.